ADC10D1000, ADC10D1500
SNAS462Q –OCTOBER 2008–REVISED MARCH 2013
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DES/Non-DES Mode
The ADC10D1000/1500 can operate in Dual-Edge Sampling (DES) or Non-DES Mode. The DES Mode allows
for one of the ADC10D1000/1500's inputs to be sampled by both channels' ADCs. One ADC samples the input
on the rising edge of the sampling clock and the other ADC samples the same input on the falling edge of the
sampling clock. A single input is thus sampled twice per clock cycle, resulting in an overall sample rate of twice
the sampling clock frequency, e.g. 2.0/3.0 GSPS with a 1.0/1.5 GHz sampling clock. See Dual Edge Sampling
Pin (DES) for information on how to select the desired mode. Since DES Mode uses both I- and Q-channels to
process the input signal, both channels must be powered up for the DES Mode to function properly.
In Non-ECM, only the I-input may be used for the DES Mode input. In ECM, either the I- or Q-input may be
selected by first using the DES bit (Addr: 0h, Bit 7) to select the DES Mode. The DEQ Bit (Addr: 0h, Bit: 6) is
used to select the Q-input, but the I-input is used by default. Also, both I- and Q-inputs may be driven externally,
i.e. DESIQ Mode, by using the DIQ bit (Addr: 0h, Bit 5). See THE ANALOG INPUTS for more information about
how to drive the ADC in DES Mode.
The DESIQ Mode results in the best bandwidth. In general, the bandwidth decreases from Non-DES Mode to
DES Mode (specifically, DESI or DESQ) because both channels are sampling off the same input signal and non-
ideal effects introduced by interleaving the two channels lower the bandwidth. Driving both I- and Q-channels
externally (DESIQ Mode) results in better bandwidth for the DES Mode because each channel is being driven,
which reduces routing losses (increases bandwidth).
In the DES Mode, the outputs must be carefully interleaved in order to reconstruct the sampled signal. If the
device is programmed into the 1:4 Demux DES Mode, the data is effectively demultiplexed by 1:4. If the
sampling clock is 1.0/1.5 GHz, the effective sampling rate is doubled to 2.0/3.0 GSPS and each of the 4 output
buses has an output rate of 500 MSPS. All data is available in parallel. To properly reconstruct the sampled
waveform, the four bytes of parallel data that are output with each DCLK must be correctly interleaved. The
sampling order is as follows, from the earliest to the latest: DQd, DId, DQ, DI. See Figure 7. If the device is
programmed into the Non-Demux DES Mode, two bytes of parallel data are output with each edge of the DCLK
in the following sampling order, from the earliest to the latest: DQ, DI. See Figure 8.
The performance of the ADC10D1000/1500 in DES Mode depends on how well the two channels are
interleaved, i.e. that the clock samples either channel with precisely a 50% duty-cycle, each channel has the
same offset (nominally code 511/512), and each channel has the same full-scale range. The ADC10D1000/1500
includes an automatic clock phase background adjustment in DES Mode to automatically and continuously adjust
the clock phase of the I- and Q-channels, which also removes the need to adjust the clock phase setting
manually. A difference exists in the typical offset between the I- and Q-channels, which can be removed via the
offset adjust feature in ECM, to optimize DES Mode performance. If possible, it is recommended to use the Q-
input for better DES Mode performance with no offset adjustment required. To adjust the I- or Q-channel offset,
measure a histogram of the digital data and adjust the offset via the Control Register until the histogram is
centered at code 511/512. Similarly, the full-scale range of each channel may be adjusted for optimal
performance.
Sampling Clock Phase Adjust
The sampling clock (CLK) phase may be delayed internally to the ADC up to 825 ps in ECM. This feature is
intended to help the system designer remove small imbalances in clock distribution traces at the board level
when multiple ADCs are used, or to simplify complex system functions such as beam steering for phase array
antennas.
Additional delay in the clock path also creates additional jitter, so a clock jitter-cleaner is made available when
using the sampling clock phase adjust, see LC Filter on Sampling Clock. Nevertheless, because the sampling
clock phase adjust delays all clocks, including the DCLKs and output data, the user is strongly advised to use the
minimal amount of adjustment and verify the net benefit of this feature in his system before relying on it.
LC Filter on Sampling Clock
A LC bandpass filter is available on the ADC10D1000/1500 sampling clock to clean jitter on the incoming clock.
This feature is only available when the CLK phase adjust feature is also used. This feature was designed to
minimize the dynamic performance degradation resulting from additional clock jitter as much as possible. It is
available in ECM via the LCF (LC Filter) bits in the Control Register (Addr: Dh, Bits 7:0).
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